Method for destaticizing polyethylene and articles thereby obtained



METHOD FDR DE'sTA'TIcIzING PDLYETHYLENE AND ARTICLES THEREBY OBTAINEDFiled April 30. 1956 April 29, 1958 w E wALLEs 2,832,699

HTORA/EVS United States Patent O METHD FOR DESTATHCIZRNG POLYETHYLENEAND ARTICLES THEREBY OBTAINED Wilhelm E. Walles, Midland, Mich.,assignor to The Dow Chemical Company, Midland, Mich., a corporation ofDelaware Application April 39, 1956, Serial No. 581,556

18 Claims. (Cl. 117--69) This invention relates to a method fordestaticizing polyethylene and shaped articles comprised of polyethyleneso as to overcome their propensity for accumulating static electricalcharges. It also has reference to the destaticized polyethylene articlesthereby obtained.

Polyethylene has many attractive properties which make it desirable foremployment in various shaped articles including fibers, filaments,yarns, threads (and fabrics constructed therefrom), ribbons, tapes,foils, films, sheets, molding and the like. Shaped articles frompolyethylene, however, exhibit an inordinate capacity for accumulatingsurface charges of static electricity. This characteristic makes themextremely difcult and unwieldly to manipulate and handle during variousmanufacturing operations and use applications. It also lessens theirattractiveness in other aspects for being employed for many purposes.For example, many individuals may object to the delitescent electricalshocks they may be subject to or the sparks that may be discharged whenI they serve as the effective grounding means for polyethylene articleswhen there is a considerable accumulation of electrical charges on thearticles. Furthermore, electrostatically charged polyethylene articlesdisplay great tendencies to collect dust and dirt and to haveundesirably high degrees of soil retentivity. This of course, limitstheir adaptability for being utilized in a completely satisfactorymanner for many upholstery, furnishing, decorative and other uses.

There are many known materials that have been proposed and claimed aspossessing the capability, when applied as an antistatic agent, todestaticize synthetic hydrophobic polymeric materials includingpolyethylene. Most of the known antistatic agents and materials arecomprised of molecules that contain both hydrophilic and hydrophobicunits or portions therein. The hydrophilic portion of the molecule insuch antistatic agents is designed and adapted to attract or retainmoisture, or both, which increases the electrical conductivity orreduces the electrical resistivity of the surface on which the agent isattached. The hydrophobic portion of the molecule, which is generallymore similar to the usually hydrophobic materials, such as polyethylene,that are desired to be destaticized, is intended to provide theanchoring or attachment of the agent on the surface through the physicalinfluence of so-called Van der Waals forces or the like in order topermit the desired conductivityincreasing function of the hydrophilicportions of the antistatic agents molecule to exert its beneficialeffect. As is generally comprehended, a surface which is relatively moreelectrically conductive (or less resistive) is usually more antistaticin nature in that it tends less to accumulate charges of staticelectricity.

While it would be highly advantageous to effectively and in asubstantially permanent manner provide antistatic agents and materialson the surfaces of polyethylene articles, great difficulty has beenencountered in achieving such a desideration through the practice ofheretofore known techniques. The considerable chemical inertness ofpolyethylene and the relatively smooth, sleek and impervious surfaceswhich occur on the vast majority of polyethylene articles usuallyprevent suitable chemical bonding or physical attachment, or both, frombeing obtained with the antistatic agents that may be applied thereto.As a consequence, most antistatic agents and materials are notsufficiently substantive for or adherescent on polyethylene articles toavoid having an undesirable susceptibility for being removed uponexposure to various solvents, including water and organic materials.

It would be advantageous and extremely beneficial to providepolyethylene surfaces with hydrophilic substituents that wereeffectively and substantially permanently attached without beingdependent upon mere physical attractions for such purposes or withoutrequiring to be applied while in molecular connection with a hydrophobicunit that is adapted to be physically attracted to the siuface. In thisway, the usually encountered shortcomings and deficiencies ofconventional antistatic agents could be avoided for purposes ofdestaticizing polyethylene. v

Therefore, it is among the principal objects of the present invention toprovide an improved destaticizing method for polyethylene articles sothat they might be less propense to develop static electric charges. Itis also an object of the invention to provide a method for destaticizingpolyethylene articles so that the benefits of such treatment areeffectively and substantially permanently retained by the articledespite rigorous exposure to washing and cleaning with water and organicmaterials and other solvent exposures. It is among the ancillary objectsof the invention to provide destaticized polyethylene articles whichresult from practice of the method. It is a predominating object of thepresent invention to accomplish these intendments without detractionfrom or deletion of the other desirable characteristics and propertiesof polyethylene.

According to the present invention, all or any desired portion of thesurface of polyethylene or a polyethylene article may be destaticized bya method which comprises subjecting the surface of the polyethylene orpolyethylene article to the action of a sulfonating reagent selectedfrom the group consisting of concentrated sulfuric acid containing atleast about 98 percent by weight of H2804, oleum, anhydrous solutions ofoleum and free sulfur trioxide and subsequently applying to the therebysulfonated surface an agent consisting of a hydrophilic compound that isfree from hydrophobic groups and contains a functional basic atom in itsmolecular arrangement that, preferably, is a nitrogen atom.Advantageously, the sulfonated surface may be washed free from excesssulfonating reagent prior to the application of the hydrophilic agent.Practice of the method of the invention, if carefully conducted, has noadverse effect on pigments and other additaments such as stabilizers andthe like which may be dispersed throughout the polyethylene for variouspurposes. After application of the hydrophilic agent which exerts adesired antistatic influence on the surface of the polyethylene articleto which it is firmly attached, the destaticized product may be employedsatisfactorily in any application wherein it is desirable for it to besubstantially free from surface accumulation of static electricalcharges. The functionally basic hydrophilic agent, which, preferably,contains a functionally basic nitrogen atom, is elfectively andsubstantially permanently retained on destaticized polyethylene articlesin accordance with the invention.

While the surface sulfonation of the polyethylene may be accomplishedwith any of the sulfonating agents of the invention, it is usually moreadvantageous to employ oleum (which is sometimes known as fumingsulfuric acid) containing from trace amounts to about 10 percent byweight of dissolved, free sulfur trioxide. If desired, the oleum canalso be employed beneficially in anhydrous solutions with othermaterials such as acetic anhydride and the like which permit effectivequantities of thesulfur trioxide to be available in an amount which isequivalent to that which is provided in the oleum. in certain instances,it may be convenient to employ free sulfur trioxide vapors which,beneficially, may be diluted to a concentration `as low as lO percent orless, for example, with a suitable inert gas such as nitrogen.

Generally, a satisfactory degree of surface sulfonaticn may be obtainedwhen operating at temperatures between the freezing point of thesulfonating reagent and about 150 C. for periods of time ranging frommatters of merely several seconds or even almost instantaneous periodswhich involve mere fractions of :seconds to hours. Frequently, when anoleum is employed which contains dissolved, free sulfur trioxide inamounts ranging from traces to about l percent 'oy weight, the surfacesulfonation may be performed suitably at an operating temperature ofabout 50 C. within a time period of about five minutes.

The degree .of surface sulfonation which is obtained on the polyethylenearticle predetermines the relative quantity of the hydrophilic agentthat may be effectively and permanently attached thereto upon itssubsequent application and, for all practical purposes, precurses theresults which may be realized by practice of the invention. rPhe degreeof sulfonation that may be obtained in any particular instance issomewhat interdependent on the nature of the polyethylene and thespecific physical form or structure of the polyethylene article that isbeing treated, the strength or effective sulfur trioxide concentrationof the reagent, the operating temperature and the length of thetreatment. Care should be taken to avoid sulfonation conditions whichmay be too strong or vigorous, as may occur when an oleum is employedWith a relatively high 'free sulfur trioxide content at too high atemperature or for .too long a period of time. Care should also be takento employ more moderate treating conditions upon more delicatestructures such as fine filaments or fibers and the like. If suchprecautions are not assiduously observed, the polyethylene article maybe caused to decompose and degrade resulting in its being darkened anddiscolored to an intolerable extent. Conversely, optimum destaticizingresults may not be obtained if overly weak sulfonating conditions areemployed which may not sufficiently modify the surface of thepolyethylene article to permit a suitably beneficial quantity of theanti-static agent to be effectively and permanently applied thereto.

The functionally basic hydrophilic agent that may ybe employed in thepractice of the invention may consist of any type of hydrophilic unitthat may be desired and that is free from hydrophobic groups. As isapparent, units that have Imore definitely pronounced hydrophilictendencies, such as those that may contain a plurality of hydroxyl andequivalent substituents, usually secure a greater destaticizing benefiin the practice of the invention, frequently with application ofrelatively smaller quantities, than when less hydrophilic units areinvolved. Preferably, as has been indicated, the basic functionality ofthe hydrophilic agent is derived from a functionally basic nitrogen atomthat is contained in the molecule with the hydrophilic unit as invarious amines and the like and in compounds having quaternary ammoniumgroups such as in alkyl pyridines and the like. If desired however, thebasic functionality of the agent may be derived from other than nitrogenatoms as in certain quaternary compounds of phosphorous, arsenic andantimony and in certain ternary sulfur compounds. It may also be derivedfrom oxygen atoms having certain structural characteristics as arepresent in particular ether linkages. Advantageously, a hydrophilicamine that is free from hydrophobic groups and contains any beneficialhydrophilic unit in its molecule may be utilized. It is generallydesirable Cir for the hydrophilic agent to have relatively pronouncedhydrophilic characteristics as may be obtained when it contains ahydrophilic unit in the molecule that consists of at least about 3hydrophilic rnoups such as hydroxyl groups. This more readily secures amarked desta-tici'zing effect in the polyethylene article uponapplication of the hydrophilic agent to the sul'fonated surface. Somedegree of destaticization may be obtained, however, with such relativelylow molecular weight amine hydrophilic agents as diethanolamine,triethanolamine, diethylene triamine and the like.

Polyglycol amines, particularly those having a molecular weight of atleast about may usually be employed with especial advantage ashydrophilic agents in the practice .of the invention. Such polyglycolamines, for example, as glucosamine or as contain in their molecules atleast about 3 to 4 recurring glycol units from ethylene or propyleneoxide, or both, such as the polyglycol amines which may be obtained fromThe Dow Chemical Company under the trade-designations Dowpolyglycolamine 3F57 and Dow polyglycolamine 3F59 or under thetrade-name Dowanol 22 Amine, may generally be utilized with exceptionalbenefit.

Since the application of the hydrophilic agent on the sulfonatedpolyethylene surface is essentially in the nature of a metatheticalreaction, there is little criticality involved in the conditions of itsemployment with respect to time, temperature and concentration. It isusually beneficial and convenient, however, to apply it from arelatively low concentration dispersion or solution, as between about land 5 percent by weight, in a suitable liquid medium such as water. Thismanner of employment permits immersion of the sulfonated polyethylene inan applicating bath of the hydrophilic agent although, if desired, otherapplication techniques may also be utilized for the hydrophilic agent.The extremely effective and substantially permanent retention of thehydrophilic agent by the polyethylene in the practice of the inventionis due to the bonding ionic attraction involved between the former andthe sulfonated surface of the latter. In practical effect, as it were,the method of the invention destaticizes the hydrophobic polyethylenearticle by a superficial provision of the `necessary hydrophilicmaterial Without involving or requiring extraneous and externalhydrophobic matter.

Any desired degree of destaticization of the polyethylene article may beeffected. Generally, within the limits of beneficial operatingconditions, longer and more intense sulfonations permit the subsequentapplication of greater amounts of the hydrophilic agent to yieldproductshaving greater degrees of antistatic characteristics. It is possible,for example, to prepare a polyethylene article that is sufficientlydestaticized so as to be substantially completely antistatic incharacter. Advantageously, the invention may be practiced withfabricated articles including fibers, filaments, yarns, threads,ribbons, tapes, foils, films, sheets, moldings and the like and onmaterials constructed therefrom such as cloth and fabric from textilefibers, filaments and the like of polyethylene. As indicated, an entiresurface need not be destaticized on such fabricated articles as filmsand moldings or on other articles. lf it is not desired or required toobtain the benefit of a completely destaticized surface, only certainpreferred areas or portions of a surface may be treated in accordancewith the invention.

In the accompanying drawing there is schematically illustrated a sheetin Figure l and a filament in Figure 2 as lbeing typical polyethylenearticles whose surfaces may advantageously be destaticized in accordancewith the present invention.

By way of exemplary illustration, a colored fabric woven from pigmentedpolyethylene monofilament was immersed in a 3 percent oleum (whichcontains about 3 percent by weight of free sulfur trioxide dissolved inH2804) at a temperature of about 50 C. for about five aaeaooe minutes.After being sulfonated, the fabric was washed thoroughly in water andthen immersed in a 2 percent by weight aqueous solution of glucosaminewhich was maintained at a temperature of about 50 C. ri'he immersion wascontinued for about live minutes before the treated fabric was removedfrom the solution the hydrophilic agent, washed thoroughly with waterand dried. its surface resistivity was then tested to determine itsantistatic characteristics by tautly connecting a sample of thedestaticized fabric between two electrodes, each of which were 7centimeters long, spaced parallel 4 centimeters apart and across whichthere was applied a 500 volt direct current potential. The test wasconducted at room temperature under about 80 percent relative humidity.The surface resistivity of the destaticized fabric was found to be about101@ ohmcentimeters. in comparison, untreated polyethylene fabric of thesame type has a surface resistivity between about 1012 and 101L ohmcentimeters. The destaticized fabric retained its antistatic characterwithout appreciable alteration even after being subjected to water atabout 55 C. for 1/2 hour and to a five minute immersion in acetone.

Similar good results were obtained when 2 percent by weight aqueoussolutions of Dow polyglycolamine 3F57, Dow polyglycolamine 3F59 andLDowanol 22 Amine were employed as hydrophilic agents in place ofglycosamine. When diethanolamine was employed in the same manner as ahydrophilic agent in place of glucosamine, the antistatic character ofthe destaticized fabric was reduced in the neighborhood of 10 percent(expressed in surface resistivity value) from the foregoing. Whendiethylene triamine, triethanolamine, monoisopropanolamine and guanidinecarbonate were employed, a destaticizing effect was obtained althoughits magnitude was much less pronounced than that achieved withglucosamine.

Similar excellent results may be obtained when other sulfonatingconditions within the scope of the invention are employed on thepolyethylene and when other polyglycolamines and other functionallybasic nitrogen atomcontaining and other hydrophilic materials areutilized.

1t is to be fully understood that the present invention is to beconstrued and interpreted not by the foregoing didactic description andspecication but in the light of what is set forth and defined in theappended claims.

What is claimed is:

1. Method for destaticizing polyethylene and polyethylene articles whichcomprises subjecting the surface of the polyethylene to a sulfonatingreagent which is selected from the group consisting of concentratedsulfuric acid containing at least about 98 percent by weight of H2804,oleum, anhydrous solutions of oleum and free sulfur trioXide; andsubsequently applying to the sulfonated surface an agent consisting of ahydrophilic compound that is free from hydrophobic groups and contains afunctional basic atom in its molecular arrangement.

2. The method of claim 1 wherein the sulfonating reagent is comprised ofoleum.

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3. The method of claim 1 wherein the sulfonating reagent is comprised ofoleum that contains from trace amounts to about 10 percent by weight ofdissolved, free sulfur trioxide.

4. The method of claim 1 wherein the polyethylene is subjected to thesulfonating reagent at a temperature between the freezing point of thereagent and about C.

5. The method of claim 1 wherein the sulfonating reagent is comprised ofoleum that contains from trace .t amounts to about 10 percent by weightof dissolved, free sulfur triide and the polyethylene is subjected tothe olcurn at a `temperature of about 50 C. for about tive minutes.

6. The method of claim 1 and including the steps of washing thesulfonated article free from the sulfonating reagent before applying thehydrophilic agent, washing the article after application of thehydrophilic agent, and drying the destaticized article.

7. The method of claim 1 wherein the hydrophilic agent consists of ahydrophilic compound that contains a functional basic nitrogen atom inits molecular arrangement.

-8. The method of claim 1 wherein the hydrophilic agent is a hydrophilicamine.

9. The method of claim 1 wherein the hydrophilic agent is a polyglycolamine.

10. The method of claim 1 wherein the hydrophilic agent is glucosamine.

11. Destaticized solid polyethylene having a sulfonated surface to whichis applied and bonded an agent consisting of a hydrophilic compound thatis free from hydrophobic groups and contains a functional basic atom inits molecular arrangement.

12. The destaticized polyethylene of claim 11 wherein the hydrophilicagent consists of a hydrophilic compound that contains a functionalbasic nitrogen atom in its molecular arrangement.

13. The destaticized polyethylene of claim 11 wherein the hydrophilicagent is a hydrophilic amine.

14. The destaticized polyethylene of claim 11 wherein the hydrophilicagent is a polyglycolamine.

15. The destaticized polyethylene of claim 11 wherein the hydrophilicagent is glucosamine.

16. A shaped polyethylene article in accordance with claim l1.

17. A polyethylene lm in accordance with claim 11.

18. A polyethylene lilamentary article in accordance with claim 1l.

References Cited in the tile of this patent UNiTED STATES PATENTS Re.24,062 Horton Sept. 20, 1955 2,255,940 Rogers Sept. 16, 1941 2,400,720Staudinger May 21, 1946 2,576,980 Treue Dec. 4, 1951 2,626,876 CarnesJan. 27, 1953 2,727,831 Dixon Dec. 20, 1955 OTHER REFERENCES Vojulskiiet al.: Doklady Akad. Nauk. S. S. S. R. 73, 747-50 CA 45: 899e.

1. METHOD FOR DESTACTIZING POLYETHYLENE AND POLYETHYLENE ARTICLES WHICHCOMPRISES SUBJECTING THE SURFACE OF THE POLYETHYLENE TO A SULFONATINGREAGENT WHICH IS SELECTED FROM THE GROUP CONSISTING OF CONCENTRATEDSULFURIC ACID CONTAINING AT LEAST ABOUT 98 PERCENT BY WEIGHT OF H2SO4,OLEUM, ANHYDROUS, SOLUTIONS OF OLEUM AND FREE SULFUR TRIOXIDE, ANDSUBSEQUENTLY APPLYING TO THE SULFONATED SURFACE AND AGNET CONSISTING OFA HYDROPHOLIC COMPOUND THAT IS FREE FROM HYDROPHOBIC GROUPS AND CONTAINSA FUNCTIONAL BASIC ATOM IN ITS MOLECULAR ARRANGEMENT.